short cycling inefficiency

AlexR

Lots of people mention short cycling as being ineffcient.  Can anyone quantify that?  I assume it depends on the boiler, but knowing something like "it works at XX% efficiency for the first YY seconds and then ZZ after that" or "it ramps up from XX to YY percent over ZZ minutes" would be useful in figuring out how much I care about short cycling.

nicholas bonham-carter

cost of short-cycling

the most correctable short-cycling is that which is caused by inadequate main [not radiator] venting, or which can result from a mis-set thermostat anticipator. these are well worth fixing.

other causes which are harder to correct include an over-sized boiler.

the loss of efficiency is hard to calculate, but should be easy to feel in terms of comfort. basically you are burning more gas while waiting to get steam to the radiators in a series of on-off steps. this probably results in a wider swing of temperatures in your house.--nbc

TomM

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thinking out loud here:

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find the heat loss of (boiler + water + returning condensate) for the time that your burner is off , then figure out how much energy is needed to heat that back up to boiling,

VS.

the energy required to produce steam while its already steaming. 



runtime is not really a factor, because your burner will run the same time whether its one cycle to thermostat satisfaction or ten cycles to thermostat satisfaction.  It can only produce steam while the burner is on, and a given amount of steam is needed to fill the radiators and release enough latent heat to satisfy the thermostat, so there is no benefit to short cycling.   the burner will have the same runtime in both cases.  But in a short cycling case, it has to do more work to heat up cooled down water in the boiler. 

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Correct me if i'm wrong. 

Mike Kusiak_2

Yes, to a point

The real cause of cycling inefficiency is the standby heat loss of the boiler while not firing. Because of imperfect insulation of the jacket and loss of heat from hot air in the combustion chamber going up the flue, the boiler temperature will drop between firings. This heat loss has to be made up by increased firing time. The more often the boiler cycles and the shorter the cycles, the more significant the loss becomes. The burner also takes some time to achieve efficient combustion after first firing. In general a long continuous burn is more efficient than a number of shorter ones.



This loss becomes especially significant with a power burner which initiates a pre and post purge of the combustion chamber on each firing cycle. Cold air is blown through the combustion chamber before and after firing to clear the chamber. During rapid cycling on pressure, this loss of heat can become appreciable.

Jean-David Beyer

I guess that varies depending on boiler design.

My boiler holds only three quarts of water. The burner is inside the heat exchanger, and the heat exchanger is not even insulated. The heat exchanger is inside the plastic box of the boiler, so any heat lost only slightly pre-heats the combustion air.



Now the control also continues to run the circulator after the fire is turned off. Default is 30 seconds, but for my radiant floor, it is set to run for 90 seconds and for the indirect hot water heater, 120 seconds. By watching the input and output temperatures, I see that this effectively empties the hot water from the heat exchanger. So the heat loss from the exchanger is pretty low.



This is no excuse for allowing rapid cycling, because of the wear-and-tear factor, but it does mean less heat leaks out into the garage where the boiler is located, or out the exhaust pipe.

Jamie Hall

Indeed...

There are really two kinds of "short cycling", and it is really important to distinguish between them.  The first has to do with the thermostat and the building.  The idea here is to control the system so that the building -- and the folks in it -- maintain a reasonably constant temperature.  Reasonably being determined by a whole range of factors!  Typically for a steam system holding a constant temperature (disregarding set backs, which is a whole 'nother can of worms) the thermostat should call for heat about once an hour, at the most.  That can be set at the thermostat.



Then there is what I think of as "real" short cycling -- where the burner runs for a few minutes, and shuts off for a minute or so, and comes back on.  This is inefficient indeed, and is to be avoided.  There are basically two causes for this -- either the boiler is making steam faster than the system can condense it, or the boiler is making steam faster than the air can get out of the system.  The second cause, if it's the only problem, is easy to fix -- more vents! -- and will show up, usually, near the beginning of a cycle.  The first cause is a bit harder to fix.  To a certain extent it is inherent in boilers which have only one firing rate -- which is most of them; if the boiler is big enough to heat up all the piping in addition to the radiation (that pickup factor thing) it may be too big towards the later stages of a long run, when everything is already nice and hot.  At that point, the pressure will rise and the boiler will cycle on pressure.  The more closely the boiler and firing rate are matched to the system, the less of a problem this will be -- and may not show up at all, or only when recovering from a moderate to deep setback or on a really cold day.  However, if the boiler or firing rate is significantly too large for the system, this type of short cycling will happen sooner, and will happen for most of the times the boiler runs.  Not good.  It may be possible to reduce this problem by downfiring the burner, but there is a definite limit on that.



There is another cause of short burner runs which isn't often considered, and is particularly true for vapour and other very low pressure systems.  These use a vapourstat for control, and a vapourstat is a very senstive device.  In some cases there is enough random minor fluctuation of steam pressure in a system to cause the vapourstat to trip before it should.  This can be cured by simply installing a snubber between the pigtail and the vapourstat.

haaljo

I asked on controls about an ancient

fully modulating gas valve mentioned by a retired mechanical I know that he had 10 years ago on his old hot water boiler. "It was probably a Honeywell Modutrol" and adjusted natural gas and air as to load.  My friend got kind of dreamy eyed when he mentioned that 2 cubic foot box. He did not remember it's name.but he smelled gas coming out of it so it was time to replace the heating system.

Why not anything like that today for residential?

haaljo

I asked on controls about an ancient

fully modulating gas valve mentioned by a retired mechanical I know that he had 10 years ago on his old hot water boiler. "It was probably a Honeywell Modutrol" and adjusted natural gas and air as to load.  My friend got kind of dreamy eyed when he mentioned that 2 cubic foot box. He did not remember it's name.but he smelled gas coming out of it so it was time to replace the heating system.

Why not anything like that today for residential?

haaljo

I asked on controls about an ancient

fully modulating gas valve mentioned by a retired mechanical I know that he had 10 years ago on his old hot water boiler. "It was probably a Honeywell Modutrol" and adjusted natural gas and air as to load.  My friend got kind of dreamy eyed when he mentioned that 2 cubic foot box. He did not remember it's name.but he smelled gas coming out of it so it was time to replace the heating system.

Why not anything like that today for residential?

haaljo

Opps sorry multiple posts Admin fix pse

Thanks

haaljo

Opps sorry multiple posts Admin fix pse

Thanks

haaljo

"My boiler holds only three quarts of water"

When I read that I thought there was a new steam boiler design but then you mentioned radiant heating loop and I don't think you are using steam for that?

Also, your concern for wear and tear may be somewhat important but it's more of an efficiency thing.

Jean-David Beyer

Not steam.

Sorry for the confusion. My boiler is hot water, not steam. I forgot that this thread is in the Strictly Steam area.



The thermostats in each zone are set to cycle once an hour. This is important for the zone that is radiant on slab. The other zone is baseboard, where I have installed a lot more than would normally be required so I can run it at lower return temperatures. (Today it was running about 126F supply and return I do not know, but obviously it was less.)



I used to have a conventional boiler (non mod|con, no reset). The boiler would go on for about 45 seconds and off for 75 to 90 seconds as I recall. I assume that is considered rapid cycling. It was about 55 years old when I got rid of it in May 2009. Actually, that was after I reduced the rapid cycling. I do not know what it was before. There was an aquastat in there that its hysteresis set to something like 5 degrees. This was adjustable, so I set it to 15 degrees or so, but that was a bad idea. Expansion tank was too small, so the relief valve would dribble. I cut it back to 10 degrees and everything was OK. It still rapid cycled, but the thing (an old 1950s GE) was built like a tank and survived. Was still running OK, but who knows what efficiency it was.



My radiant zone does not rapid cycle. On really cold days, it will run for 18 hours at a time (outdoor reset). My baseboard does rapid cycle, but I improved it a lot by setting the hysteresis from 10F to 15F for that zone, and I lowered the maximum firing rate from 94% to 55%. The control system is too fast, so temperatures tended to increase way too fast, and by the time the control realized it, it was so hot it shut down the firing. Then it cooled off, and did it again. It would go through a cycle in about 5 minutes. With the lower firing rate, the control could keep better track of water temperature and control it better. When just that zone is calling for heat, it is oversized and the boiler will not modulate down to the actual load. But now a cycle lasts about 15 minutes, and will probably get longer as the temperature goes down outside.



I do not think I would lose much efficiency by rapid cycling with this boiler (other than the losses involved in getting the firing going).  I doubt very much goes up the exhaust when it is not firing because the blower is off except when it fires, and as the lower the demand, the slower the blower goes. And because of the way the controls work, not much hot water remains in the boiler when it is not firing.



So for me, I think the main problem with rapid cycling is due to wearout of the components. With conventional boilers, having the hot water sitting around in the boiler and heating up the air going out the stack could pay a large efficiency penalty, but for me, essentially nothing goes in the air intake pipe or out the exhaust if that blower is not running.



As far as new steam boiler design, there is an old design that is interesting. I am pretty sure it was never used for home heating though. It is my understanding that one model of the Stanley Steamer boiler let a measured drip of water hit a hot plate that flashed to steam. (Flash boiler.) That way the steam generation rate could be quickly veried to comply with the varying steam needs to drive the thing. I do not know if they used distilled water in it, but I imagine that would be a good idea.

Jamie Hall

Old coal fired boilers

sometimes were fully modulating on pressure -- with no electrical controls involved at all.  As long as the building super (e.g. me!) kept coal in the thing, it would hold the design pressure within an ounce or two.  Lovely mechanical contraption -- if a bit Heath Robinson -- involving basically a water manometer with a float, and a set of levers and chains and balances which moved the main underfire air damper in response to pressure -- as the pressure rose, the damper closed and vice versa.  On a Hoffman Equipped system, but the same basic principle was used on a lot of vapour systems.



Worked fine, so far as heat was concerned.  I don't suppose anyone ever measured the efficiency or CO levels in the stack at nearly closed damper conditions, though, and I have a suspicion that they were pretty horrible!

AlexR

interesting...

Thanks for all the replies.  Our boiler is oversized for the radiators (350 EDR vs 190 [excluding pickup], though it's closer if you include the 40' of baseboard hot water that we don't generally have on), so we get short cycling after 45 or 50 minutes of firing. 



It's interesting that standby heat loss is the primary inefficiency of short cycling and yet standby heat loss is also a reason to have a deep setback and fewer cycles per day.  I think I'm going to have to crunch some numbers to see whether the short cycling is better or worse than a shallow setback.

nicholas bonham-carter

crunched numbers

let us know what the result of your crunching is.

there is more to read on the subject if you do a search here for "setback" or "short-cycling"--nbc